1. Field Of The Invention
The present invention relates in general to methods and apparatuses for interfacing holographic and electronic data, especially to methods and apparatuses for reading a holographic wavefront and translating the wavefront into a digital electronic signal.
2. Description Of Prior Art
Holography is a form of three-dimensional photography in which coherent light is used in combination with ordinary photographic plates to produce images which can be viewed in three dimension without special optical equipment. Preparation of the hologram involves the channeling of the coherent light to illuminate both the subject and the photographic plate. An interference pattern is produced on the photographic plate which shows no resemblance to the original until it is developed and illuminated from behind by similar coherent light. The process was discovered in the late 1940s, and with the advent of the laser which provides a highly coherent light source, many significant advances have been made in this field.
The crucial feature of coherent waves that is essential to holography is that two or more intersecting coherent wave trains can from standing wave patterns. When two trains of coherent light waves intersect in a given region, there will be places in that region where the electric field values add up to a greater value that either wave train alone. The will also be regions where the electric field values subtract. At the area of intersection of such light trains, a definite pattern of light and dark areas is present.
A characteristic of interference patterns of periodic coherent waves is that this spatial overlap of light intensity remains fixed and constant while the waves overlap. This is referred to as a standing wave pattern. Exposure of a photographic plate in this region of overlap leads to a hologram.
The present invention is a method and apparatus for reading a holographic wavefront and translating this wavefront into a digital electronic signal, and for propagating the same or a different wavefront from such a signal. Conventionally, a holographic wavefront is produced by the interference of a coherent light reference beam with an object split from the same laser emission and modified by interaction with the object or with the process being holographed.
Heretofore, methods and apparatuses to record the resulting wavefront rely on projecting the wavefront onto a recording medium of sufficient definition to record the resulting interference pattern. The interference pattern consists of bright regions where the two beam's wavefronts interfere constructively with one another and dark regions where the waves interfere destructively and cancel each other. Such regions are referred to as fringes. Such fringes range from almost parallel to the recording medium surface, as in the case of a reflection hologram, to essentially perpendicular to it in the case of a transmission hologram. It is with an interference pattern of the latter variety with which the current invention is envisioned to have the most ready application.
Conventional approaches to the recording an interference pattern utilize a wide variety of light sensitive emulsions. In the case of a transmission hologram the holographic wavefront can be reconstructed by introducing a display beam which corresponds in both angle of incidence and wave length to the original reference beam, which is projected onto the holographic storage medium. Typically, the image bearing wavefront is then reconstructed from the diffraction caused by the display beam's passage through the stored interference fringes. The image produced is three-dimensional and possesses a very high information content.
A number of significant limitations arise from such conventional approaches which greatly limit and restrict holographic application. Of primary significance is that storage and reproduction mediums are fixed photo-chemical processes. This severely restricts the reproduction of holograms and limits them to a single hologram per recording medium per cycle and, for all practical purposes, restricts holographic reproductions to those that can produced by the mechanical means described above. Though computers can, and have been applied to compose fringe patterns of hypothetical object wave fronts, and holographic television has been proposed, there is, until the instant invention, no practical and efficient means of interfacing these or any other electrical devices, with the actual light waves.